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NASA continues to publish some interesting air quality research. Take the recent summary of research on the interactions of ozone and sulfate in air pollution and climate change by Dr. Nadine Unger and colleagues. It demonstrates the contribution of ozone to the formation of sulfates, which have severe adverse effects on air quality and moderate the effects of global warming. The take home message needs to be that, when we model the future CO2 emission reductions required to slow climate change, we need to assume that we will effectively control sulfate emissions -- so we need to reduce CO2 emissions even more. The mistaken message that can be heard is that emission of sulfate precursors emissions is really "good."

In two recent studies, we describe how emission of ozone precursor
gases (gases which react to form ozone) can dramatically affect both
air quality and climate forcing by increasing the levels of
tropospheric sulfate. Like many of their precursors, ozone and sulfate
are pollutants that can detrimentally affect climate, agriculture, and
human health. However, they act differently on the climate, as ozone
tends to warm the planet while sulfate cools it.

Ozone and sulfate aerosol are formed in the atmosphere from chemical
reactions involving gases such as sulfur dioxide, carbon monoxide and
methane, which are emitted by both natural and human sources, the
latter including automobile traffic, power generation, industry and
agriculture.

Many of the reactions and molecules involved in the formation of
sulfate and ozone overlap. Sulfate is generated by the oxidation of
sulfur dioxide by the hydroxyl radical or by hydrogen peroxide, both of
which can be derived from ozone. Likewise, ozone production requires
the presence of nitrogen oxides, which sulfate can remove by conversion
to nitric acid.

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In the future, man-made emissions of the precursor gases will change
as more nations industrialize, other nations implement emissions
control strategies, and world population grows, leading to changes in
the amount of pollution that people are exposed to. We used the GISS
ModelE to simulate a future Earth atmosphere based on a
middle-of-the-road projection of man-made precursor emissions to
simulate levels of air pollution in the future and to investigate how
the interaction between sulfate and ozone might affect future climate
changes.Figure 1 shows the percentage change in annual average sulfate
aerosol and ozone air pollution at the Earth's surface by 2030. There
are large increases in pollution in subtropical regions, especially
Asia. Over the Indian subcontinent the surface sulfate aerosol amount
changes from around 400 pptv in the present day to around 2000 pptv at
2030 and the surface level ozone increases from around 35 ppbv to 60
ppbv. The potential consequences of such large increases in the sulfate
aerosol and ozone pollution may have serious social and economic
impacts across the Indian subcontinent.

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Next we calculated how much of the sulfate aerosol increase is due
to the change in ozone precursor emissions alone and find the influence
to be surprisingly large. Figure 2 shows the amount of the future
surface sulfate aerosol that comes from changes in ozone precursor
emissions only. Increases in ozone precursor emissions contribute about
10% to surface sulfate increases over the Middle East, North Africa and
the most developed parts of South America, but the largest influence
occurs over the Indian subcontinent, where the surface sulfate is 20%
greater as a result of the future emissions-driven increases in ozone
precursors.

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Moreover, ozone precursor emissions also contribute 20% of the
negative sulfate forcing over India, which is more than twice the
direct positive forcing of ozone itself. In contrast, changes in
sulfate precursor emissions do not significantly affect future ozone
levels.

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This new insight, that ozone precursors have a surprisingly large
influence on air quality via sulfate and that their overall climate
impact may be opposite to the conventional view, is of direct relevance
to regulatory policy. The interconnection between ozone and sulfate can
complicate environmental efforts, as a reduction of ozone precursors
would improve surface air quality, but also impose additional positive
forcing via sulfate reduction. Our results suggest that future
regulations should address ozone and sulfate simultaneously, which they
do not currently do, as well as consider both air quality and climate.